As discussed in our earlier application, oxygen gas sensors containing solid electrolyte oxygen gas sensor elements are used to measure the oxygen content of an automotive exhaust gas for the purpose of regulating the efficiency of the engine through control of the air to fuel ratio. These generally thimble-shaped sensor elements having an inner conductive catalyst electrode on the inner surface of the thimble and an outer conductive catalyst electrode on the outer surface of the thimble are conductively connected to a monitoring and actuating system to adjust said air-fuel ratio.
In our earlier application, the use of a chemical treatment, wherein the inner electrode was contacted with an inorganic acid or acid salt, produced a chemical treatment of the inner electrode and resulted in an increased voltage output in the positive range for the sensor element and also reduced the internal resistance of the solid electrolyte sensor element, both of which are beneficial to the operation of the sensor. Also, as discussed therein, when the chemically activated sensor elements are also subjected to a current treatment, wherein the sensor element is subjected to a direct current, with the outer catalytic electrode as a cathode, and at an elevated temperature and in the presence of a reducing gas, the above properties are further enhanced and, in addition, the switching response time required for switching from rich to lean gas composition readings is reduced.
In using the combined chemical and current activation treatment of our previous application, however, the need for the presence of a reducing gas at the outer electrode during current activation was present, as well as the need for a recovery period during which the sensor element was maintained at the elevated temperature, in order to provide a stable condition within the solid electrolyte body.
We have now discovered that if current activation of the sensor element is combined with the chemical activation, where the current activation is carried out by applying a direct current to the sensor element, with the outer electrode as an anode, only a nonoxidizing gas need be present and, in addition, the need for a recovery period is removed, where short time periods of current application are used.